Method of forming an electrode casing for an alkaline electrochemical cell with reduced gassing

ABSTRACT

Electrochemical cells including a casing or cup for direct electrical contact with a negative electrode or counter electrode and serving as the current collector for the electrode. The casing includes a substrate having a plated coating of an alloy including copper, tin and zinc, the coating having a composition gradient between the substrate and the external surface of the coating wherein the copper content is greater adjacent the substrate than at the external surface of the coating and the tin content is greater at the external surface of the coating than adjacent the substrate. Methods for forming a coated casing and an electrochemical cell including a coated casing are disclosed, preferably including providing an electrode casing with a coating utilizing variable current density plating that reduces discoloration of a surface exposed to the ambient atmosphere.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 11/933,552, filed Nov. 1, 2007, which claims the benefit ofU.S. Provisional Application No. 60/855,876, filed Nov. 1, 2006, both ofwhich are fully incorporated herein by reference.

BACKGROUND

This invention relates to electrochemical cells with aqueous alkalineelectrolytes and a negative electrode or counter electrode in contactwith a cell casing or cup. Electrochemical cells can be used asbatteries to provide energy to operate electronic devices.Electrochemical cells, particularly prismatic cells and button cells,are suitable for applications including hearing aids, sensors,computers, calculators, watches and other devices.

Embodiments include zinc/metal oxide cells, with positive electrodescontaining one or more metal oxides such as silver oxide, manganesedioxide, nickel oxyhydroxide, silver copper oxide and silver nickeloxide. Other embodiments include cells with catalytic electrodes, suchas electrodes that reduce oxygen or generate oxygen or hydrogen, such asfuel cells, metal-air cells, oxygen generating cells and hydrogengenerating cells. The invention also relates to methods for forming anelectrochemical cell, preferably including providing an electrode casingwith a coating that reduces discoloration of a surface exposed to theambient atmosphere.

Other embodiments of electrochemical cells include alkaline cells havinga metal, such as zinc, as an active negative electrode material and apositive electrode including a metal oxide or metal dioxide such assilver oxide and manganese dioxide. Cells containing a metal oxideand/or metal dioxide are usually noted for good voltage stability duringdischarge. Zinc/metal oxide and zinc/metal dioxide cells typicallyinclude an alkaline electrolyte and an electrolyte permeable separatorfilm located between the negative electrode and the positive electrode.Examples of such cells are described in U.S. Pat. Nos. 4,405,698,6,794,092, 6,759,166, and 6,723,469, as well as U.S. patent applicationSer. No. 11/731,361 filed Mar. 30, 2007, all fully incorporated hereinby reference.

Some electrochemical cells can be used to generate gases, such as oxygenand hydrogen. One type of electrochemical battery cell has a metal, suchas zinc, as an active negative electrode material within the cell anduses a gas from outside the cell, such as oxygen contained in the air,as an active positive electrode material. An example of such a cell isan alkaline electrolyte zinc/air cell. When an external electricalcircuit is completed, oxygen in air that enters the cell is reduced atthe catalytic positive electrode to produce hydroxyl ions. The hydroxylions migrate to the negative electrode where they oxidize the zinc,producing electrons that flow through the external circuit. An exampleof an alkaline zinc/air cell button cell is disclosed in U.S. Pat. No.6,602,629, which is hereby incorporated by reference. Electrochemicalcells similar in design to an alkaline zinc/air cell can also be used toproduce oxygen or hydrogen. In an oxygen generating cell, the catalyticelectrode is the oxygen generating electrode, and the counter electrodeis a metal oxide (e.g., manganese dioxide, silver oxide, mercuric oxideor nickel oxyhydroxide) rather than zinc. When an electric current isforced to flow through the cell, the metal oxide is reduced to a loweroxidation state, and oxygen is evolved at the oxygen generatingelectrode. Such a cell is described in U.S. Pat. No. 5,242,565, which ishereby incorporated by reference. In a hydrogen generating cell, thecatalytic electrode is the hydrogen generating electrode; the activematerial of the counter electrode can be a metal such as zinc. Whenoxygen is excluded from the cell and the cell is short circuited,hydrogen is produced. Examples of hydrogen generating electrochemicalcells are disclosed in U.S. Pat. Nos. 5,242,565, 5,707,499 and6,060,196, all of which are hereby incorporated by reference.

The generation of hydrogen within alkaline electrochemical cells duringperiods of storage and non-use can be detrimental, particularly forcells in which a portion of the cell housing serves as a currentcollector. Mercury can be added to these cells to reduce suchundesirable hydrogen gassing; however, the addition of mercury isundesirable for health and environmental reasons. Efforts have been madeto eliminate mercury, but the cells are still more susceptible togassing without added mercury.

Electrochemical cells, particularly prismatic cells and button cells,can comprise two electrodes, each in contact with an electrode casingthat can serve as a current collector therefore. Depending on the cellconstruction, a casing can be referred to as a negative electrodecasing, a positive electrode casing, a catalytic electrode casing or acounter electrode casing. Such casings can have similarly shaped bodiessuch as a cup or a pan, each with a closed end and an open end generallyopposite the closed end. Of course, metal-air, fuel and gas generatingcells typically include one or more orifices in the catalytic electrodecasing to allow ingress or egress of gases such as oxygen, carbondioxide and hydrogen. Prismatic cells and button cells can be sold assingle cell batteries not having any jackets or labels covering theexternal surfaces of the cells. Accordingly, any portion of the cell'sinternal components, such as liquid electrolyte or salt, that contactthe external surfaces of the cells, such as through leakage, is presenton an external surface of the one or more cell casings. The presence ofleaked components on the external cell surfaces can lead to corrosionthereof. In some instances, the ambient atmosphere can also lead todiscoloration of the external cell surfaces. It is also desirable tomaintain good electrical contacts in other single cell batteries as wellas cells used in multiple cell batteries.

Various attempts have been made to minimize discoloration of externalsurfaces of electrochemical cells and to improve the leakage resistanceof cells which can affect tarnishing of the cells external surfaces. Forexample, clad metals, such as triclad nickel/stainless steel/copper(Ni/ss/Cu), have been utilized as negative electrode casings for buttoncells including zinc/silver oxide, zinc/manganese dioxide, zinc/nickeloxyhydroxide and metal-air cells. The nickel plated outer layer isbelieved to provide an attractive appearance while resistingdiscoloration. The stainless steel provides strength and the copperinner layer has desirable electrical conductivity, and provides acontinuous coating over the stainless steel that can be formed into adesired shape without cracking to expose the stainless steel layertherebeneath. The copper layer is also readily plated with zinc whencontacted by a negative electrode containing an alkaline electrolyte andzinc as an active material.

U.S. Pat. No. 6,830,847 relates to a zinc/air button cell comprising acathode casing and an anode casing wherein the anode casing is insertedinto the cathode casing. The anode casing is formed of multi-clad metallayers, for example nickel/stainless steel/copper. A reportedlyprotective metal is plated on the exposed peripheral edge of the anodecasing. The protective metal is desirably selected from copper, tin,indium, silver, brass, bronze or gold. The application of protectivemetal covers the multi-clad metals exposed along the peripheral edgesurface. The protective metal is also desirably plated onto the portionof the outside surface of the anode casing abutting the insulatingmaterial placed between the anode and cathode casing. Application of theprotective metal to the anode casing peripheral edge reportedlyeliminates the potential gradients caused by exposure of the differentmetals comprising the multi-clad material. This reportedly reduces thechance of electrolyte leakage which can be promoted by secondaryreactions occurring along the anode casing peripheral edge.

U.S. Patent Application Publication No. 2003/0211387 relates to agalvanic element with an alkaline electrolyte and a zinc negativeelectrode, in a housing in the form of a button cell, where at least theouter surface of the cell's cap is coated with a Cu—Sn-alloy containingno nickel or with a Cu—Sn—Zn-alloy containing no nickel. The alloycontains about 20% to about 90% Cu, preferably about 50% to about 60%Cu, with the remainder being Sn, or about 50% to about 60% Cu and about25% to about 35% Sn, with the remainder being Zn.

U.S. Patent Application Publication No. 2006/0246353 relates to anelectrochemical cell with a zinc-containing negative electrode, anaqueous alkaline electrolyte and a cup-shaped metal negative electrodecasing in contact with the negative electrode. The negative electrodecasing is formed from a substrate that is substantially free of copperand at least those portions of the surface of the negative electrodecasing in the seal area and the current collector area are coated with alayer of an alloy comprising copper, tin and zinc. The alloy layerreduces hydrogen gassing within the cell and is particularly useful incells with no added mercury. Embodiments of the invention include cellswith prismatic, cylindrical and button shaped containers and cells withpositive electrode active materials including manganese dioxide, silveroxide and oxygen.

Taking the above approaches into consideration, there is still a needfor electrochemical cells having constructions which provide leakageresistance and include a negative electrode or counter electrode casingthat serves as a current collector, and further resist discoloration,thereby contributing to a desirable aesthetic appearance of theelectrochemical cells. It is also desirable for the cells to haveexcellent shelf life, electrical characteristics and discharge capacity.

In view of the above, an object of the present invention is to provideelectrochemical cells that contain no added mercury and are lesssusceptible to hydrogen gassing during storage and periods of non-usethan cells according to the prior art.

Another object of the invention is to provide an electrochemical cellwith an electrode containing a metal such as zinc as an active material,and an aqueous alkaline electrolyte that contains no added mercury andproduces little hydrogen gas during storage and periods of non-use.

Yet another object of the invention is to provide a method of making anelectrochemical cell with an electrode containing a metal such as zincas an active material, and an aqueous alkaline electrolyte that containsno added mercury and produces little hydrogen gas during storage andperiods of non-use.

A further object of the invention is to provide an electrochemical cellwith no added mercury and a method of making the cell, the cell havingan aqueous alkaline electrolyte and a housing with a negative casing incontact with a negative electrode in which the cell is economical toproduce, exhibits a low level of hydrogen gassing during storage anduse, and has an attractive external appearance.

Still another object of the invention is to provide a casing for anelectrochemical cell, particularly a counter electrode or negativeelectrode casing, having a coating including an external layercomprising copper, tin and zinc at least on the outer surface of thecasing that is exposed to the ambient environment that provides thecasing with resistance to discoloration.

Another object of the present invention is to provide an electrochemicalcell with a negative electrode or counter electrode casing having acoating of two or more layers on a substrate including a copper innerlayer, with the coating comprising a first layer having a greater weightpercentage of copper than a second layer and the second layer having agreater weight percentage of tin than the first layer, wherein the cellhas no added mercury, with the exposed external surface of the coatingcasing having a bright finish resistant to discoloration.

Still another object of the present invention is to provide a metalcasing which serves as the current collector for a negative electrode orcounter electrode that is post-plated with at least a first layer and asecond layer on at least an exposed portion of an exterior surface ofthe casing wherein the first layer has a higher copper content than thesecond layer, and the second layer has a higher tin content than thefirst layer.

Yet another object of the present invention is to provide a method ofmaking an electrochemical cell with a negative electrode or counterelectrode casing comprising a plated coating having variable tin contentand copper content in different layers or depths of the coating that arederived from utilizing a variable current density during plating.

A further object of the present invention is to provide a method forforming a coated casing for an electrochemical cell including utilizingmultiple current densities to provide various coated layers on thecasing.

SUMMARY

The above objects are met and the above disadvantages of the prior artare overcome by an alkaline electrochemical cell with two electrodes,one of which is disposed in and in direct contact with one part of themetal housing, which serves as the current collector for that electrodeand is made from a material, in one embodiment a copper clad material,that is coated with an alloy comprising copper, tin and zinc. Thecoating has a composition gradient between the clad material and theexternal surface of the coating. In some embodiments the copper contentis greater adjacent to the clad material substrate (i.e., at theinternal surface of the coating) than at the external surface of thecoating. In some embodiments the tin content is greater at the externalsurface of the coating than at the internal surface of the coating. Thecomposition gradient can be a uniform or non-uniform gradient. In anembodiment with a non-uniform gradient, the coating can comprisemultiple layers, each having a different average composition. Thecoating can be deposited on a surface of the substrate, such as theinterior surface or the exterior surface, or both the interior surfaceand exterior surface of the substrate using an electrolytic platingprocess, and the composition gradient can be produced by varying theplating current during the plating process.

In one aspect of the invention, an electrochemical cell is disclosed,comprising a first and a second electrode, a separator disposed betweenthe first electrode and the second electrode, an aqueous alkalineelectrolyte, and a housing comprising a first electrode casing and asecond electrode casing and an electrically insulating gasket disposedbetween the first electrode casing and second electrode casing, thehousing containing the first electrode, second electrode, separator andelectrolyte wherein the first electrode casing is in contact with thefirst electrode and has an interior surface and an exterior surface andcomprises a substrate coated with at least a first layer and a secondlayer disposed on the first layer, wherein the first layer and secondlayer, independently, comprise copper, tin and zinc, wherein the firstlayer has a greater weight percentage of copper than the second layer,and wherein the second layer has a greater weight percentage of tin thanthe first layer.

In another aspect of the invention, an electrochemical cell isdisclosed, comprising a first and a second electrode, a separatordisposed between the first electrode and the second electrode, anaqueous alkaline electrolyte, a housing comprising a first electrodecasing and a second electrode casing and an electrically insulatinggasket disposed between the first electrode casing and second electrodecasing, the housing containing the first electrode, second electrode,separator and electrolyte, wherein the first electrode casing is incontact with the first electrode and has an interior surface and anexterior surface and comprises a substrate coated with at least a firstlayer and a second layer disposed on the first layer, wherein the firstlayer and second layer, independently, comprise copper, tin and zinc,wherein the first layer has a greater weight percentage of copper thanthe second layer, and wherein the second layer has a greater weightpercentage of tin than the first layer, and the second layer has a ratioof surface values measured according to XPS of 73 to 84 atomic percentcopper, 11 to 17 atomic percent tin, and 4 to 10 atomic percent zinc.

In yet another aspect of the invention, an electrochemical cell isdisclosed, comprising a catalytic electrode for reducing oxygen orgenerating hydrogen or oxygen, a counter electrode comprising a metal asan active material, an aqueous alkaline electrolyte, a separatordisposed between the catalytic electrode and the counter electrode, ahousing comprising a catalytic electrode casing and a counter electrodecasing; and an electrically insulating gasket disposed between thecatalytic electrode casing and counter electrode casing, the housingcontaining the catalytic electrode, counter electrode, separator andelectrolyte, wherein the counter electrode casing is in contact with thecounter electrode and comprises a plated substrate having an interiorsurface and an exterior surface, wherein the substrate is plated with atleast a first layer and a second layer disposed on the first layer,wherein the first layer and second layer, independently, comprisecopper, tin, and zinc, wherein the first layer has a higher coppercontent than the second layer, and wherein the second layer has a highertin content than the first layer.

In yet still another aspect of the invention, an electrochemical cell isdisclosed, comprising a catalytic electrode for reducing oxygen orgenerating hydrogen or oxygen, a counter electrode comprising a metal asan active material, an aqueous alkaline electrolyte, a separatordisposed between the catalytic electrode and the counter electrode, ahousing comprising a catalytic electrode casing and a counter electrodecasing, an electrically insulating gasket disposed between the catalyticelectrode casing and counter electrode casing, the housing containingthe catalytic electrode, counter electrode, separator and electrolyte;wherein, the counter electrode casing is in contact with the counterelectrode and comprises a plated substrate having an interior surfaceand an exterior surface, wherein the substrate is plated with at least afirst layer and a second layer disposed on the first layer, wherein thefirst layer and second layer, independently, comprise copper, tin, andzinc, wherein the first layer has a higher copper content than thesecond layer, and wherein the second layer has a higher tin content thanthe first layer; and wherein the second layer has a ratio of surfacevalues measured according to XPS of 73 to 84 atomic percent copper, 11to 17 atomic percent tin, and 4 to 10 atomic percent zinc.

In yet another aspect of the invention, a method for forming a coatedcasing for an electrochemical cell is disclosed, comprising the steps ofproviding an electrode casing comprising a metal substrate having aninterior surface and an exterior surface, plating a first metal layerwith a plating solution on at least the exterior surface of thesubstrate that is adapted to be exposed to the atmosphere when assembledin an electrochemical cell utilizing a first current density, andchanging the first current density to a second different current densitywhile the electrode casing is in contact with the plating solution andplating a second metal layer on the first layer.

In yet a further aspect of the invention, a method for forming anelectrochemical cell is disclosed, comprising the steps of providing afirst electrode casing comprising a metal substrate having an interiorsurface and an exterior surface, plating the first electrode casingutilizing a variable current density while the first electrode casing isin contact with a plating solution thereby producing a plated casing,and wherein the tin content increases from a portion of the plating incontact with the substrate when compared to the plating exposed on thesurface of the casing and forming an electrochemical cell comprising afirst electrode in contact with the interior surface of the firstelectrode casing and an aqueous alkaline electrolyte, wherein prior toplating the first electrode casing has a copper layer on the interiorsurface.

These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

Unless otherwise specified, the following definitions and methods areused herein:

A cell containing no added mercury is one in which mercury is notintentionally added to the cell or any cell component, and any mercurypresent in the cell is found only in very small amounts, typically lessthan 50 parts per million by weight, desirably less than 10 parts permillion, preferably less than 5 parts per million, and more preferablyless than 2 parts per million, as an impurity or contaminant. U.S. Pat.No. 6,602,629 to Guo et al., herein incorporated by reference, disclosesthe method used to determine the total level of mercury in a cell.

For all test methods utilized herein, one or more alternate test methodscan be utilized if the values obtained by the alternate methods areconsistent with the values obtained by the disclosed methods, and thuslie within the given range of values for the disclosed test methods.

Unless otherwise specified herein, all disclosed characteristics andranges are as determined at room temperature (20-25° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features andadvantages will become apparent by reading the detailed description ofthe invention, taken together with the drawings, wherein:

FIG. 1 is an elevational view, in cross-section, of a button-shapedhydrogen generating cell;

FIG. 2 is an enlarged cross-sectional view through the materialconstruction of the cup of the cell in FIG. 1 taken at line 2-2,illustrating one embodiment of a clad material including a coatingthereon;

FIG. 3 is a schematic illustration of one embodiment of a spouted bedelectrode plating device, including a plating chamber, for use inplating cups of the present invention;

FIG. 4 is an elevational view, in cross-section, of a button-shapedelectrochemical cell; and

FIG. 5 is an enlarged cross-sectional view through the materialconstruction of the cup of the cell in FIG. 4 taken at line 5-5,illustrating one embodiment of a clad material including a coatingthereon.

DESCRIPTION

Electrochemical cells of the present invention can include button-typecells, prismatic cells, and cylindrical-type cells, wherein a negativeelectrode, anode or counter electrode casing is in contact with thenegative electrode, anode or counter electrode, and thereby functions asthe respective electrode current collector. A button cell is a smallround cell having an overall height that is less than its diameter. Acylindrical cell is a round cell having a straight cylindrical body andan overall height that is greater than its diameter. A prismatic cell isa non-round cell having a generally prismatic cross section, for exampletriangular, rectangular, trapezoidal, hexagonal; examples of whichinclude flat, rectangular and square cells. The cell can utilize anyelectrochemical system for which it is desirable to plate the negativeelectrode or counter electrode with a layer comprising copper, tin andzinc. Preferred cell types are those that contain an aqueous alkalineelectrolyte, such as alkaline cells that contain zinc as a negative orcounter electrode active material, and cells that include, for example,one or more of manganese dioxide, silver oxide and a catalytic materialfor reduction of oxygen from outside the cell as a positive electrodeactive material. In one embodiment, the cell is a fluid-depolarized cellwherein the fluid utilized by the cells is preferably a gas such asoxygen. Fluid-depolarized electrochemical cells can include metal-aircells, such as zinc-air cells, or fuel cells.

Electrochemical cells of the present invention in various embodimentscan also include a gas generating or catalytic electrode that producesgas by oxidation or reduction at the catalytic electrode. Examples ofgasses capable of being generated by gas generating electrode includeoxygen and hydrogen. For example, a gas generating cell can be provided,such as an alkaline cell having a counter electrode comprising a metaloxide, wherein, when current is forced to flow through the cell, themetal oxide is reduced to a lower oxidation state or the correspondingmetal, and oxygen is evolved at the gas generating electrode. In thecase of an embodiment of a hydrogen generating cell wherein hydrogen isevolved from the gas generating electrode, such as in an alkaline cell,oxygen is excluded from the gas generating electrode and hydrogen gas isgenerated within the gas generating electrode when an electric currentis enabled to flow through the cell. Further explanations regarding gasgenerating cells and gas generating electrodes, and materials therefore,are set forth in U.S. Pat. Nos. 5,242,565; 5,707,499 and 6,060,196,herein fully incorporated by reference.

An embodiment of the invention is a metal/air cell as illustrated inFIG. 1, which is a cross section of a button-shaped cell. FIG. 4illustrates a further embodiment of a button-shaped electrochemical cellaccording to the present invention. Button cells are generallycylindrical in shape and have maximum diameters that are greater thantheir total heights. The maximum diameter is generally between about 4mm to about 35 mm, desirably from about 5 mm to about 35 mm. Preferablythe maximum diameter is no greater than about 30 mm and more preferablyno greater than about 20 mm. The button cells have a maximum height orthickness, measured perpendicular to the diameter, generally from about1 mm to about 20 mm, desirably from about 1 mm to about 15 mm.Preferably the maximum height is no greater than about 10 mm and morepreferably no greater than about 8 mm. While FIGS. 1 and 4 each refer toa button-type cell, it is to be understood that the invention is notlimited thereto, and includes other cell types such as describedhereinabove.

In FIG. 1, the cell 10 has a two-part metal housing containing acatalytic positive electrode 20 and a negative counter electrode 28,separated by an electrically insulating, ion permeably separator 24. Onepart of the metal housing, referred to as the casing or can 12, is inelectrical contact with the catalytic electrode 20, and the other partof the housing, referred to as the casing or cup 26 is in contact withand serves as the current collector for the counter electrode 28.Between the can 12 and the cup 26 is an electrically insulating grommetor gasket 30. The rim 34 of the can 12 is deformed inward and downwardagainst the gasket 30 and cup 26 so that the gasket 30 provides acompression seal between the can and cup side walls 16 and 36,respectively. The gasket 30 has an inward extending base and a lip 32that form a groove into which the bottom rim 35 of the cup 26 fits. Theaxial force applied by the bent rim 34 of the can 12 also creates a sealbetween the cup bottom rim 35 and the periphery of the inside surface ofthe can bottom 14. At least one aperture 18 is located in the can bottom14 through which air containing oxygen used an active material by thecatalytic electrode 20 can enter the cell 10. The catalytic electrode 20includes a catalytic mixture with a metal screen or expanded metalcurrent collector embedded in the surface facing the separator 24. Thecatalytic electrode 20 also includes a hydrophobic membrane 22 laminatedto the side of the catalytic mixture opposite the separator 24. Locatedbetween the hydrophobic membrane 22 and the inside surface of the canbottom 14 is a gas diffusion layer 38.

A gas generating cell can have a construction similar to that of cell 10in FIG. 1. The catalytic electrode 20 can be either a positive electrodeor a negative electrode, and the counter electrode 28 can be either anegative or a positive electrode. In one embodiment of a gas generatingcell, the gas diffusion layer 38 can have a high hydrogen permeabilityand a low oxygen permeability to allow hydrogen gas generated at thecatalytic electrode 20 to escape from the cell through the aperture 18while substantially blocking the entry of air from the externalenvironment. An example of a suitable material for the hydrogendiffusion layer 38 is sintered polytetrafluoroethylene, such as virginsintered PTFE membrane conforming to ASTM D3308 Type II and SAE AMS3662C, an example of which is D/W 202 skived PTFE from DeWal Industries,Saunderstown, R.I., USA, 0.051 mm (0.0020 inch) thick and having typicaltensile strength of 422 kg/cm² (6000 pounds per square inch), a typicalelongation of 325 percent, a typical dielectric strength of 2000 volts,a typical BJH adsorption cumulative pore volume of 0.0004 to 0.0023cm³/g and a typical density of 2.1 to 2.2 g/cm³.

FIG. 4 illustrates cell 210 of a similar construction to cell 10 shownin FIG. 1, and includes a two-part metal housing including a positiveelectrode casing or can 212, housing a positive electrode 220, and anegative counter electrode casing or cup 226 housing negative counterelectrode 228 in contact therewith. The negative electrode cup 226serves as the current collector for the negative electrode 228. Thepositive electrode casing 212 is in electrical contact with the positiveelectrode 220. Gasket 230 is disposed between can 212 and cup 226 withrim 234 of can 212 deformed against gasket 230 and cup 226, therebyproviding a compression seal between the can 212 and cup 226 sidewalls216 and 236, respectively. As indicated above, gasket 230 has an inwardextending base and a lip 232 that forms a groove into which the bottomrim 235 of the cup 226 fits. Unlike cell 10, cell 210 has no aperture inbase 214. Positive electrode 220 includes one or more active materials.In one embodiment the active material is manganese dioxide, in anotherembodiment the active material is silver oxide (such as monovalentsilver oxide and/or divalent silver oxide), and in yet anotherembodiment the active material is a mixture of manganese dioxide andsilver oxide. A separator 224 is disposed between the positive electrode220 and the negative electrode 228.

In some embodiments, a refold anode cup such as cup 226 in FIG. 4 isutilized instead of a straight-walled anode cup such as cup 26 inFIG. 1. In both types, terminal ends 35, 235 define the opening in anodecups 26, 226. As known in the art, refold anode cups have rounded rimsthat are substantially U-shaped at the ends that define the openings inthe anode cups. A refold anode cup is formed in one embodiment byfolding a portion of the wall of the cup back upon itself so that theopening in the cup is defined by a folded rim. The refold anode cup 226can be formed of the materials, dimensions and the like as describedherein with respect to anode cup 26.

The cup 26, 226 forms the top of the cell 10, 210 when the cell ispositioned as shown in FIGS. 1 and 4. It is made from a metal substratewith sufficient mechanical strength for the cup to withstand cellclosing and sealing forces and maintain an adequate seal with the gasket30, 230 and can 12, 212, even with the internal pressure that may bepresent in the cell 10, 210 during normal use. The cup 26, 226 has aninterior or internal surface exposed to and having a portion in contactwith the counter electrode 28, 228 and/or electrolyte. Cup 26, 226 alsohas an exterior or external surface exposed to the ambient environmentoutside of the cell 10, 210. The material of the substrate also has goodelectrical conductivity, since the cup 26, 226 serves as the currentcollector for the counter electrode 28, 228 and an external contactterminal for the cell 10, 210. The outer surface of the substrate hasgood corrosion and discoloration resistance in environments in which thecell 10, 210 will be used. In one embodiment, in order to minimizeunwanted gas generation where electrolyte comes in contact with the cup26, 226, the substrate has an essentially continuous layer of copper onthe surface forming the inside of the cup 26, 226.

The electrochemical cell 10, 210 according to the invention includes acup 26, 226 that has a coating of one or more layers or regionsdeposited on the substrate in any suitable manner. A preferred method ofthe invention utilizes an electrolytic plating process to produce thecoating on the substrate. For added protection against externalcorrosion and internal gassing, the cup 26, 226 can be post-plated(plated after forming the substrate into a cup shape) with acopper-tin-zinc alloy, that desirably coats all exposed surfaces of theformed cup 26, 226. The cup 26, 226 can be pre-plated in anotherembodiment, wherein prior to forming the substrate into the shape of acup, one or more surfaces of the substrate are plated with a depositedcoating of one or more layers. When a pre-plated cup is cut from alarger substrate, such as a sheet, strip or roll of material, andsubsequently formed into the cup, the cut edges of the cup do notinclude the coating. In some embodiments, the cup 26, 226 can be bothpre- and post-plated. The plating of the cup 26, 226 also provides thecell 10, 210 with an attractive appearance. The term “plating” utilizedherein means electrolytic plating.

In a preferred embodiment, the substrate of cup 26, 226 is a clad metalincluding a layer of copper. The clad copper layer provides a continuouslayer of copper. The copper layer is advantageous for a number ofreasons. It has a relatively high hydrogen overvoltage, so if thecopper-tin-zinc alloy plating is not continuous or is damaged duringcell manufacturing, electrolyte will not be in direct contact with ahigh gassing, low hydrogen overvoltage metal. Copper is also relativelyductile, preventing the risk of cracking during the cup forming processto expose the lower hydrogen overvoltage metal layer beneath it. A cladmaterial is preferred because the copper layer is continuous, and thecomposition of the layer to which it is clad can be selected to providethe required strength to the substrate. Preferably the copper layer isclad to a layer of steel, such as a mild steel, a cold rolled steel or,more preferably, a stainless steel. The substrate material can be abiclad material, or it can have three or more layers, such as a tricladmaterial. Nickel is used in various embodiments as a layer of a biclador polyclad material. The nickel provides lower electrical contactresistance with the contact terminal surface of the cup 26, 226. Therelative thicknesses of the layers and the total thickness of thesubstrate material can be selected to provide the best combination ofstrength, gassing resistance and corrosion resistance, based on the cellsize and ingredients. A preferred triclad material is a nickel-stainlesssteel-copper triclad material. The stainless steel of the preferrednickel-stainless steel-copper triclad material provides structuralrigidity and is present at a thickness of generally greater than 50%,desirably between about 60% to about 92%, and preferably at a thicknessof about 85% based on the total thickness of the particular anode cup26, 226 utilized. The thickness of the nickel layer ranges generallyfrom about 2% to about 20%, desirably from about 3% to about 10%, andpreferably from about 3% to about 8% based on the total thickness of theanode cup 26. The thickness of the copper layer ranges generally fromabout 5% to about 40%, desirably from about 6% to about 30%, andpreferably from about 8% to about 15% based on the total thickness ofthe anode cup 26, 226. All values listed are prior to post-plating. FIG.2 is a cross-section of the cup 26, shown in FIG. 1, taken at line 2-2,and FIG. 5 is a cross-section of the cup 226 shown in FIG. 4, taken atline 5-5, made from a preferred triclad substrate. The substrate has astainless steel base layer 44, 244, an inner clad copper layer 46, 246and an outer clad nickel layer 42, 242. Another preferred tricladmaterial is a copper-stainless steel-copper triclad plate.

In a preferred embodiment, a coiled strip of the substrate material isformed into the desired shape using a stamping process using three ormore progressively sized stamping dies, after which the anode cup ispunched out of the coil. Using two or less dies to form an anode casingmay contribute to undesirable cell gassing.

After the cup 26, 226 is formed from the substrate material, it ispost-plated with one or more coatings or layers of a copper-tin-zincalloy 50, 250 (FIGS. 2 and 5) to provide an interior surface layer thatwill gas less than a copper surface when in contact with the electrolyteand/or counter electrode 28, 228 and an exterior surface that isresistant to corrosion and discoloration. In the case of a pre-platedcup 26, 226, the one or more coatings are applied to one or moresurfaces of the substrate material prior to forming the cup. In oneembodiment the copper-tin alloy 50, 250 includes a first portion orlayer 52, 252 and a second portion or layer 54, 254 externallypositioned with respect to layer 52, 252 and having a higher weightpercent tin content and a lower weight percent copper content whencompared to the internal layer 52, 252. The cup 26, 226 can also beprovided with a strike layer, such as a layer of copper, to improveadhesion of the copper-tin-zinc alloy. For example, a copper strikelayer can be applied to a nickel-stainless steel-copper,nickel-stainless steel or steel cup 26, 226 to provide improved adhesionof the copper-tin-zinc layer(s) to the non-copper surface layer(s) ofthe cup 26, 226.

The composition of the copper-tin-zinc alloy can be selected to providethe desired plating coverage and adhesion, gassing and corrosionresistance, and appearance. Other metals are desirably avoided, but maybe present due to impurities in the plating bath or solution. Becausethe plating current density is lower on the interior surface than on theexterior surface of the cup 26, 226, the composition of the plated alloywill tend to be different on the interior and exterior surfaces.

In one embodiment, the interior portion or layer, such as layer 52 or252 of the plated alloy will preferably contain a ratio of 50 to 70weight percent copper; 26 to 42 weight percent tin; and 3 to 9 weightpercent zinc, measured at a central portion of, by scanning electronmicroscopy/energy dispersive X-ray spectroscopy (SEM/EDS) to provide lowgassing within the cell. The lower limit for the amount of copper ismore preferably at least 52 weight percent, and most preferably at least56 weight percent. The upper limit for the amount of copper is morepreferably at most 68 weight percent and most preferably at most 64weight percent. The lower limit for the amount of tin is more preferablyat least 28 weight percent, and most preferably at least 32 weightpercent. The upper limit for the amount of tin is more preferably atmost 39 weight percent and most preferably at most 36 weight percent.The lower limit for the amount of zinc is more preferably at least 4weight percent. The upper limit for the amount of zinc is morepreferably at most 8 weight percent and most preferably at most 7 weightpercent. The composition of the alloy plating is determined with energydispersive X-ray spectrometry (EDS) by bombarding the surface with abeam of high energy electrons in a scanning electron microscope (SEM). ALEO™ 438 VP scanning electron microscope (LEO Electron Microscopy Ltd.)and an OXFORD™ Ge-detector (Oxford Instruments PLC) with ISIS™ software(Isis Software) are suitable for SEM/EDS measurements. The measurementsare performed on the central region of the boss flat area, i.e., not ona sidewall or curved connecting area between the sidewall and centralregion of the cup, and if both the inside and outside of the cup areplated, the measurements are performed on the outside of the cup. Thecentral region is analyzed for composition of copper, tin and zinc intwo different areas and averaged. The SEM parameters used for themeasurements are: 400× magnification, 3.2 nA beam current, and 20 kVaccelerating voltage. The composition of the adjacent surface of the cupsubstrate, for example nickel, detected during measurement on theoutside central region of the plated cup, is not reported in thecomposition of the alloy. Resulting measurement values will be higherthan actual when testing a cup substrate containing one or more samemetals present in the substrate layer adjacent the plated layer. Forexample, copper measurement values will be higher than actual whentesting the inside of a cup when a Ni/ss/Cu cup is tested because of thecopper clad layer beneath the copper, tin and zinc plating layer(s).Because the copper to tin to zinc ratio determined from SEM/EDS is theaverage for the entire coating, to determine the composition of anyindividual layer, the composition(s) and thickness(es) of the previouslyplated layer(s) must be taken into account.

X-ray photoelectron spectroscopy (XPS) is useful to determine atomiccomposition surface values of the plated alloy layers of the cup 26,226. A sample to be measured is mounted on carbon tape and analyzedwith, for example a PHI® 5600 ESCA system (Physical Electronics, Inc.).Each layer can be tested prior to plating a subsequent layer. If boththe inside and outside of the cup 26, 226 are plated, the measurement ismade on the outside of the cup 26, 226. Al Ka radiation (1486.6 eV) isused to excite photoelectrons for collection by a hemisphericalanalyzer. The step size is set to 0.4 eV, and the bandpass used is 93.9eV. The elemental lines measured during depth profiling are the C1s,O1s, Cu2p3, Zn2p3 and Sn3d5. The sputter depth interval is 2.8 nm, basedon Ta₂O₅ calibration, to a total depth of 112 nm. The composition istaken at the desired depth (100 nm) and normalized with respect tocopper, tin and zinc to obtain a ratio. In one embodiment, the interiorportion or layer, such as layer 52 or 252 of the plated alloy,preferably contains a ratio of 85 to 91 atomic percent copper; 6 to 11atomic percent tin; and 2 to 6 atomic percent zinc, measured at acentral portion of the casing, by XPS. The lower limit value of copperis more preferably at least 85 atomic percent, and most preferably atleast 86 atomic percent. The upper limit value of copper is morepreferably at most 89 atomic percent and most preferably at most 88atomic percent. The lower limit value of tin is more preferably at least7 atomic percent, and most preferably at least 8 atomic percent. Theupper limit value of tin is more preferably at most 10 atomic percentand most preferably at most 9 atomic percent. The lower limit value ofzinc is more preferably at least 2 atomic percent. The upper limit valueof zinc is more preferably at most 4 atomic percent and most preferablyat most 3 atomic percent.

It may be desirable to have a white appearance on the exposed outersurface of the cup 26, 226. The lower the copper content of the platedalloy and higher the tin content, the whiter its appearance will be. Fora white color on the exterior surface of the cup 26, 226, the coppercontent on the plating surface will preferably be no more than 46 weightpercent, and if the copper content is too high, the exterior surface ofthe cup 26, 226 will be susceptible to tarnishing, especially if exposedto alkaline electrolyte (e.g., aqueous KOH), such as during assembly.Therefore, in another embodiment having a cup 26, 226 with an exteriorsurface that has a white color and is also resistant to discoloration,the outer plated layer 54, 254 preferably has a ratio of copper from 36to 46 weight percent, tin from 42 to 57 weight percent, and zinc from6.5 to 9.5 weight percent measured by SEM/EDS as described above. Thelower limit for the amount of copper is more preferably at least 38weight percent, and most preferably at least 39 weight percent. Theupper limit for the amount of copper is more preferably at most 44weight percent and most preferably at most 43 weight percent. The lowerlimit for the amount of tin is more preferably at least 47 weightpercent, and most preferably at least 48 weight percent. The upper limitfor the amount of tin is more preferably at most 52 weight percent andmost preferably at most 51 weight percent. The lower limit for theamount of zinc is more preferably at least 7.0 weight percent, and mostpreferably at least 7.5 weight percent. The upper limit for the amountof zinc is more preferably at most 9.0 weight percent and mostpreferably at most 8.5 weight percent.

In one embodiment, the exterior portion or layer, such as layer 54 or254 of the plated alloy, has a ratio of atomic composition surfacevalues of desirably from 73 to 84 atomic percent copper, 11 to 17 atomicpercent tin, and 4 to 10 atomic percent zinc, measured at a centralportion of the casing by XPS. If the cup 26, 126 is coated on both theinside and outside, the measurement is taken on the outside of the cup26, 126. The lower limit value of copper is preferably at least 75atomic percent, more preferably at least 76 atomic percent, and mostpreferably at least 77 atomic percent. The upper limit value of copperis preferably at most 83 atomic percent, more preferably at most 81atomic percent and most preferably at most 80 atomic percent. The lowerlimit value of tin is preferably at least 12 atomic percent, and morepreferably at least 12 atomic percent. The upper limit value of tin ispreferably at most 16 atomic percent and more preferably at most 15atomic percent. The lower limit value of zinc is preferably at least 5atomic percent, and more preferably at least 6 atomic percent. The upperlimit value of zinc is preferably at most 9 atomic percent.

In a further embodiment, the coating comprises one or more additionallayers located between the interior layer 52, 252 and the exterior layer54, 254 that each, independently, can have a different composition thanthe compositions of the interior and exterior layers.

In a preferred embodiment, a multi-step copper-tin-zinc alloy platingprocess is utilized. For example, the alloy plating is performed in oneembodiment at two or more different current densities, at each currentdensity for a period of time. The variable current density platingprocess can provide numerous benefits, including providing an efficientplating process in which a copper-tin-zinc alloy coating is formedhaving two or more regions or layers exhibiting differentcharacteristics. In one embodiment, a first region or alloy layer, suchas 52, 252, is plated at a relatively high current density, allowingrapid deposition of the alloy on the casing, and reducing the platingtime required, while a final step of the alloy plating process is doneat a different, preferably lower, current density to achieve a preferredcomposition in the outermost surface portion, i.e., a second region orlayer, for example 52, 254 of the total plated alloy layer composition.The variable current density plating process is particularly useful andallows the alloy layer coating composition to provide a particularsurface appearance, which may not be the best composition for resistanceto cell internal gassing. However, a bulk of the plated alloy layer,that portion below the surface portion plated at a different currentdensity, can be formed having a composition that is plated more quicklyand is more resistant to internal gassing when compared to the surfaceportion. As used herein, unless otherwise stated, current density isexpressed in units of current per total surface area of the parts beingplated.

Regarding discovery of the variable current density plating process fornegative electrode or counter electrode cups, it was observed thatunplated cups formed from triclad material of nickel, stainless steeland copper were prone to leakage. In an attempt to prevent leakage, thecups 26, 226 were plated with a copper-tin-zinc layer utilizing acurrent density of about 43 amps per square meter (4 amps per squarefoot). However, some of the cups produced exhibited an initiallyinconsistent surface color and/or undesirable surface color. It wasbelieved that the relatively high amounts of copper in the platedsurface layer led to the inconsistent color. It was discovered that bylowering current density, copper content in the plated layer decreasedand tin content in the plated layer increased. As indicated herein, byutilizing a variable current density process wherein relatively highcurrent density is utilized during a first portion of the platingprocess and a lower current density, generally below about 21.5 amps persquare meter (about 2 amps per square foot), is utilized during a finalportion of the alloy plating step, a desirable plated cup 26, 226 isconsistently formed. The variable current density plated cup exhibits afirst region or layer, located closer to the cup substrate that has agreater copper content than a second upper region or layer of theplating, with the second region or layer having a higher tin contentthan the first region or layer and thereby providing a desiredappearance on the surface that is resistant to discoloration, with thefirst region believably more resistant to internal gassing.

The total thickness of a plated copper-tin-zinc alloy coating, includingone or more layers or regions on the formed cup 26, 226 is generallyfrom 0.7 to 4.0 μm at the central portion of the cup. If both the insideand the outside of the cup 26, 226 are plated, the measurement is madeon the outside. The lower limit of the indicated total plating thicknessis desirably at least 1.0 μm and preferably at least 1.25 μm. The upperlimit of the total plating thickness is desirably at most 3.0 μm, andpreferably at most 2.5 μm. When a variable current density platingprocess is utilized, the surface or second portion or layer, such aslayer 54, 254, of the alloy forms generally about 4.0 to about 20.0percent, desirably about 6.0 to about 15.0 percent, and preferably about10.0 percent of the total thickness of the plated alloy coating on theexternal surface of the central portion of the formed cup 26, 226. Thus,the thickness of the surface portion or layer, such as 54, 254, rangesgenerally from about 0.1 to about 0.5 μm, desirably from about 0.15 toabout 0.40 μm, and preferably from about 0.20 to about 0.30 μm in oneembodiment. As indicated hereinabove, the plated alloy layer may nothave a distinct compositional boundary between layers or regions whenvariable current density plating is utilized. There may be a compositiongradient or transition region between layers or sub-layers, such asbetween layers 52 and 54, and layers 252 and 254. If the copper-tin-zincplating is too thin, there may be incomplete coverage of the cup 26,226. If the plating is too thick, the cup 26, 226, can 12 and gasket 30may not fit together properly during assembly of the cell 10. It isdesirable to provide a continuous layer of the plating over the interiorsurface of the cup 26, 226 without causing cell assembly problems.

X-ray fluorescence (XRF) can be used to determine the total thickness ofthe coating. A flat circular disc is punched from the central portion ofthe cup. The disc is irradiated using an XRF spectroscope (e.g.,available from Matrix Metrologies) that has been calibrated usingcopper, nickel and tin metal standards. The amount of tin is measuredbased on fluorescent X-rays emitted from the irradiated disc. Theoverall thickness of the coating is calculated using the measured amountof tin and the overall composition of the coating as determined bySEM/EDS (as described above). Thicknesses of the layers of the coatingcan be determined using focused ion beam microscopy (FIB). A coating ofsilicon dioxide about 5 μm thick is applied to the portion of the cup tobe tested to protect the surface. A beam of high energy Ga ions is usedto mill a trench through the copper-tin-zinc coating to create a thincross section, using a Micrion™ 2500 FIB system (Micrion Corp., now FEICompany) for example. The cup is then tilted and imaged using a scanningelectron microscope. The layers are then measured on the SEM image.

The can 12, 212 forms the bottom of the cell 10, 210 in FIGS. 1 and 4.It is made from a metal with sufficient mechanical strength for the cupto withstand cell closing and sealing forces and maintain an adequateseal with the gasket 30, 230 and cup 26, 226 even with the internalpressure that may be present in the cell 10, 210 during normal use. Thematerial of the substrate also has good electrical conductivity, sincethe can serves as an external contact terminal for the cell 10, 210. Thecan 12, 212 is made from a material that has good corrosion resistance,both in environments in which the cell 10, 210 will be used and where incontact with internal components of the cell 10, 210. An example of apreferred material is nickel plated steel.

The gasket 30, 230 is made from an elastomeric material which provides acompressive seal for the cell 10, 210. Examples of suitable elastomericmaterials include nylons. Optionally, a sealant may be applied tosealing surfaces of the gasket 30, 230, cup and/or can. Suitable sealantmaterials will be recognized by one skilled in the art. Examples includeasphalt, either alone or in combination with elastomeric materials orethylene vinyl acetate, aliphatic or fatty polyamides; and thermoplasticelastomers such as polyolefins, polyamine, polyethylene, polypropyleneand polyisobutene. A preferred sealant is SWIFT® 82996, from FoxboroAdhesives, LLC, of Research Triangle Park, N.C., USA. As an alternativeto a molded gasket 30, 230, an electrically nonconductive adhesive canbe used to seal the inner surface of the can side wall 16, 216 to theouter surface of the cup side wall 36, 236. As used herein, the cupsurface area exposed to the negative or counter electrode is thatportion of the internal surface of the cup that is not covered by thegasket or adhesive and could come in contact with the negative orcounter electrode and/or electrolyte.

The counter or negative electrode 28, 228 in one embodiment containszinc as an active material and an aqueous alkaline electrolyte. The zincis a low-gassing zinc. Examples of low gassing zincs are disclosed inU.S. Pat. Nos. 6,602,629 and 5,464,709; and U.S. Patent Publication No.2005/0106461 A1, which are hereby incorporated by reference.

A preferred zinc, as disclosed in U.S. Patent Publication No.2005/0106461 A1, is a zinc alloy powder containing bismuth, indium andaluminum, preferably about 100±25 parts per million by weight (ppm) ofbismuth, about 200±30 ppm of indium and about 100±25 ppm of aluminum.The alloy preferably contains about 35±10 ppm of lead. The preferredaverage particle size (D₅₀) for metal-air cells is less than 130microns, more preferably about 90 to about 120 microns. Preferredcharacteristics of the alloy include a tap density greater than 2.80g/cc and less than 3.65 g/cc, a BET specific surface area greater than400 cm²/g, and a KOH absorption value of at least 14 percent. Examplesof preferred zinc powders for metal-air cells are product grades NGBIA100, NGBIA 110 and NGBIA 115, manufactured by N.V. Umicore, S.A.,Brussels, Belgium, most preferably NGBIA 115. A preferred zinc powderfor zinc/metal oxide or dioxide cells such as zinc/silver oxide cells isBIA, also available from N.V. Umicore. The electrolyte can includepotassium hydroxide. In some embodiments, sodium hydroxide can replaceall or part of the potassium hydroxide.

The counter or negative electrode 28, 228 is preferably a gelledelectrode, using a binder or gelling agent. Examples of suitable gellingagents for metal-air cells include acrylic acid polymers in the 100%acid form, such as CARBOPOL® 940 and CARBOPOL® 934 from Noveon Inc. ofCleveland, Ohio, USA, and a crosslinked sodium polyacrylate, such asSANFRESH™ DK-300 and DK-500 MPS from Tomen America, New York, N.Y., USA.SANFRESH™ DK-300 is preferred. Examples of suitable gelling agents forzinc/metal oxide cells include carboxymethyl cellulose (CMC) fromHercules of Wilmington, Del., USA and a modified CMC available from FMCCorporation of Philadelphia, Pa. (USA) as AC-DI-SOLO.

The counter electrode 28, 228 can contain other ingredients, to reducegassing or to improve cell performance, for instance. Examples includezinc oxide, indium hydroxide, and one or more surfactants. A preferredsurfactant for both metal-air cells and zinc/metal oxide cells is ananionic polymer surfactant, such as DISPERBYK® D102 and D190 from BykChemie of Wallingford, Conn., USA.

The catalytic electrode 20 has a layer of a catalytic mixture, in whichthe hydrogen generating reaction takes place when the cell is a hydrogengenerating cell. The mixture can include carbon, such as PWA carbon fromCalgon Corp., Pittsburgh, Pa., USA, or DARCO® G60 carbon from AmericanNorit Co., Inc., Marshall, Tex., USA, and a catalytic material, such asa manganese oxide, held together by a binder, such as apolytetrafluoroethylene resin. The catalytic material is optional for ahydrogen generating cell but is required for an oxygen generating celland a zinc-air battery cell. This layer is somewhat porous to allow themixture to be wetted by electrolyte. Water in the catalytic electrode 20reacts to produce hydrogen gas in a hydrogen generating cell. Thecurrent collector can be an electrically conductive, corrosion resistantmetal in the form of a wire screen or expanded metal; nickel is anexample of a suitable material. The hydrophobic membrane laminated tothe catalytic layer provides a barrier to water loss from the cell orwater gain into the cell. It will have sufficient strength to withstandthe electrode lamination process and forces applied during cell closing,and it will be stable when in contact with the electrolyte.Polytetrafluoroethylene film is an example of a suitable material.

A diffusion layer 38 is located between the hydrophobic membrane 22 ofthe catalytic electrode 20 and the can bottom 14 when the cell is ahydrogen generating cell. This diffusion layer 38 has sufficientpermeability to oxygen in non-hydrogen generating cells to allow oxygenfrom outside the cell 10 to diffuse to the catalytic electrode 20through the aperture 18 in the can bottom 14. An example of a suitablematerial for the oxygen diffusion layer 38 is a PTFE film.

In embodiments of electrochemical cells having a positive electrodecontaining an active material such as silver oxide, manganese dioxide,or the like, the positive electrode can be a disk-shaped pellet moldedfrom a mixture containing active material, an electrically conductivematerial and a binder. The positive electrode pellet can be disposeddirectly against and extending across the bottom inside surface of thecatalytic electrode can 212 so that the periphery is compressed betweenthe gasket base and the inside surface of the can, as shown in FIG. 4.As silver oxide is slightly soluble in alkaline electrolyte, at leastone layer of the separator must be impermeable to silver ions. Thespecific materials and relative amounts in the catalytic electrode canvary depending upon factors including end uses. Electrically conductivematerial can be graphite, for example. The alkaline electrolytedesirably contains potassium hydroxide or both potassium hydroxide andsodium hydroxide.

A sealant such as a thermoplastic hot melt adhesive, for example SWIFT®82996 from Forbo Adhesives, LLC of Research Triangle Park, N.C., USA,can be used to bond peripheral portions of the catalytic electrode 20and the diffusion layer 38 together and/or to bond the peripheralportion of the diffusion layer 38 to the peripheral portion of the can20 to provide an improved cell seal.

A layer of porous material can be positioned between the diffusion layer38 and the central portion of the can bottom 14. If this layer isincluded, it does not extend to the peripheral portion of the diffusionlayer 38, so that the peripheral portion of the barrier membrane canseal tightly against the can bottom 14.

The separator 24, 224 is electrically nonconductive and ion permeable toelectrically insulate the catalytic electrode 20, 220 from the counterelectrode 28, 228, while allowing ions to pass through. The separator24, 224 can include one or more layers, and the layers can be the sameor different materials. A preferred separator 24, 224 has two layers,one an air-permeable, water-wettable nonwoven polypropylene film treatedwith surfactant, such as CELGARD® 5550, from Celgard, Inc., Charlotte,N.C., USA, next to the counter electrode 28, 228, and the other ahydrophobic polypropylene membrane, such as CELGARD® 3501, against thecatalytic electrode 20, 220. The layers of separator 24. 224 arepreferably adhered to the catalytic electrode 20, 220 and each otherwith an adhesive, such as a blend of carboxymethylcellulose andpolyvinylalcohol (9 to 1 by weight). In an embodiment where thecatalytic electrode includes one or more silver oxide and manganesedioxide, the separator is preferably a laminated material with layers ofcellophane and polyethylene. In a cell with a positive electrode 220comprising silver oxide, the separator 224 is resistant to silver ions.In such cells, the separator 224 can include a barrier layer such as apolyethylene/cellophane laminate impervious to silver ions but nothydroxyl ions and a soakup layer made from a material such as celluloseor polyvinyl alcohol that will retain electrolyte solution and allowhydroxyl ions to pass between the positive and negative electrodes 220and 228.

During manufacture of the cell 10, 210, the cup 26, 260 is preferablyinverted, and the components of the counter electrode 28, 280 are putinto the cup 26, 260. The components can be inserted in a two stepprocess wherein dry materials (e.g., the zinc, binder and In(OH)₃) aredispensed first, followed by the electrolyte solution, which can includeaqueous KOH solution, surfactant and zinc oxide, for example.Alternatively, the wet and dry components can be blended beforehand andthen dispensed or extruded in one step into cup 26, 226. The gasket 30,230 is then placed over the cup rim 35, 235.

The diffusion layer 38 when present, electrode 20, 220 and separator 24,224 can be inserted into the can 12, 212, which is then inverted andpressed against the cup/gasket assembly. The can rim 34, 234 is deformedinwardly, so it is compressed against the gasket 30, 230, therebysealing the cell components within the housing.

Any suitable method may be used to deform the edge of the casing inwardto seal the cell, including crimping, colleting, swaging, redrawing, andcombinations thereof as appropriate. Preferably the button cell issealed by crimping or colleting with a segmented die so that the cellcan be easily removed from the die while a better seal is produced. Asused herein, a segmented die is a die whose forming surfaces comprisesegments that may be spread apart to enlarge the opening into/from whichthe cell being closed is inserted and removed. Preferably portions ofthe segments are joined or held together so they are not free floating,in order to prevent individual segments from moving independently andeither damaging the cell or interfering with its insertion or removal.Preferred crimping mechanisms and processes are disclosed in commonlyowned U.S. Pat. No. 6,256,853, which is hereby incorporated byreference.

After a metal-air cell 10 is assembled, a suitable tab or adhesive tape(not shown) can be placed over the aperture 18, 218, when present in thecan bottom 14 until the cell 10 is ready for use.

As described above, the cup 26, 226 is plated with a layer or coating ofa copper-tin-zinc alloy after the substrate material has been formedinto the desired shape. Any suitable process can be utilized to platethe cup. A preferred process for applying the coating is electroplating.Examples of electroplating devices include, but are not limited to, abarrel plating device, a rack plating device, a vibratory platingdevice, and a spouted bed electrode plating device. The copper-tin-zincalloy can be applied directly to the formed cup substrate or at least aportion of the substrate may be coated with another metal, such ascopper, nickel or tin before the layer of copper-tin-zinc alloy isapplied to the cup. The intermediate metal coating can include one ormore metals or alloys and can be applied as an activation strike layer,for example, in order to facilitate deposition of the copper-tin-zinclayer.

In a preferred embodiment, the post-plating of cups 26, 226 is performedusing a spouted bed electrode plating device. An example of a spoutedbed electrode plating device is set forth in U.S. Pat. No. 6,193,858,herein fully incorporated by reference. A preferred spouted bedelectrode plating device is commercially available from Technic Inc. ofPawtucket, R.I. The spouted bed electrode plating device utilizessolution streams of various compositions that circulate the anode cupsin a cycle or travel path through a vessel during process steps such ascleaning, activation, plating, passivation and rinsing. The spouted bedelectrode plating device can include one or more and preferably aplurality of stations, such as but not limited to, a plating chamber, acleaning station, a rinse station, an activation station, and apassivation station that can be utilized in the anode cup tin platingprocess. The plating process is preferably conducted in a clean roomenvironment. Providing plating on the interior surface of the cup 26,226 is only preferred for zinc-metal oxide cells, especially thoseutilizing silver oxide as active material for the positive electrode.

An example of one embodiment of a spouted bed electrode plating deviceplating chamber 100 is shown in FIG. 3. Plating chamber 100 includes atank 102 having a fluid circulation receptacle 104 therein. Receptacle104 is preferably connected to the bottom of tank 102 and includes afluid aperture 106 through which a solution or fluid can pass. A vessel110 is removably connected to receptacle 104 such that fluid can passthrough fluid aperture 106 into an inlet flange 112 of vessel 110. Afterthe plating process has been performed, vessel 110 including cups 26,226, can be removed from receptacle 104 of the plating chamber 100 andtransferred to another station of the spouted bed electrode platingdevice or elsewhere for further processing.

Vessel 110 is generally a vertically extending cylinder having a conicalbottom 114. The cups 26, 226 to be plated are loaded into vessel 110.Vessel 110 is immersed in a plating solution, such as described below,contained within tank 102 at a desired level or volume sufficient toperform the plating step on the cups 26, 226, and vessel inlet flange112 is connected to fluid aperture 106 in plating chamber 100. Theplating solution is introduced into vessel 110 at a relatively highvelocity (e.g., from about 18.9 to about 56.8 liters (5.0 to 15.0gallons) per minute, preferably from about 30.3 to about 39.7 liters(8.0 to 10.5 gallons) per minute) through inlet flange 112. Gravitypulls cups 26, 226 down along conical bottom 114 and radially inwardtowards screen 116 of inlet flange 112. The cups 26, 226 in the areaabove screen 116 are forced upwards along with the plating solution intoshaft 118 in vessel 110. The cups 26, 226 and plating solution contact adeflector plate 120 located above shaft 118 and are directed radiallyoutward and downward along conical deflector 122. The cups 26, 226 falloff of conical deflector 122 and rejoin the bed of cups 26 at conicalbottom 114.

A circular anode 126 in the form of a basket or mesh surrounds vessel110 and current is conducted from anode 126 to a circular cathode ring124 through solution output screen 128 in a side wall of vessel 110 andany cups 26, 226 and plating solution therebetween. In one embodiment,the anode 126 includes titanium clad copper rods. The plating solutionis able to exit vessel 110 through the solution output screen 128 andcan be recirculated back to vessel 110. The circular anode 126 basketsurrounding vessel 110 maintains the same distance between the anodesand the cathodic moving bed of cups 26, 226, resulting in a uniformplating current density. The cups 26, 226 are plated when they settle onthe bottom of vessel 110, in direct electrical contact with the cathodering 124. The design of vessel 110 distributes the cups 26, 226 therebypreventing nesting thus providing complete plating coverage of theentire surface of each cup 26, 226. Anode basket 126 and cathode ring124 are connected to a suitable power supply and control panel.

The plating or other solution, or components of a solution, can behoused in a tank such as 109A, B or C as illustrated in FIG. 3 untilneeded. At that time, pump 108 is utilized to circulate the solutionwithin vessel 110 and/or to and from a predetermined tank or tanks, suchas 109A, B and C. One or more filters can be used to remove contaminantsfrom the plating solution. Spouted bed electrode plating chamber 100 caninclude any number of lines, pumps, filters, and valves to provide adesired process loop.

A predetermined volume and concentration of a plating solutioncontaining copper, tin and zinc is utilized in plating chamber 100. Theamount of plating solution is dependent upon factors such as thedimensions of the plating chamber 100, as well as the volume of parts tobe plated. For example, in one embodiment a volume of about 500 ml ofcups 26, 226, generally having a weight from about 218.3 to about 408.2grams (7.7 to 14.4 ounces), depending on the size of the anode cups, isutilized per batch in the plating solution. In a preferred embodiment,about 83 liters of plating solution comprising copper, tin and zincmetal salts, cyanide and caustic, one or more brightening agents andwater are utilized in plating chamber 100. The amount of platingsolution and components thereof can be adjusted to achieve desiredplating characteristics on the cups 26, 226. Plating chamber 100preferably includes a chiller in order to maintain the platingcomposition within a predetermined temperature range, as describedbelow. In a preferred embodiment, a mechanical vibrator is utilized tovibrate vessel 110 during the plating process.

The plating solution preferably comprises tin in the form of tincyanide, copper in the form of copper cyanide, and zinc in the form ofzinc cyanide, in addition to potassium cyanide, caustic, and twobrighteners, such as present in MIRALLOY® 2844 plating solution fromUmicore Galvanotechnik GmbH, Schwäbisch Gmünd, Germany. The coppercyanide is present in an amount generally from about 6 g/liter to about18 g/liter, and preferably about 8 g/liter to about 16 g/liter. The tincyanide is present in an amount generally from about 35 g/liter to about130 g/liter, and preferably from about 50 g/liter to about 110 g/liter.The zinc cyanide is present in an amount generally from about 0.40g/liter to about 1.5 g/liter, and preferably from about 0.75 g/liter toabout 1.25 g/liter.

The plating solution also can include, but is not limited to, variouscomponents such as caustic, brightening agents, antioxidants, water, orthe like.

Examples of suitable caustics include potassium hydroxide and sodiumhydroxide. In a preferred embodiment, the caustic is utilized in anamount generally from about 15 to about 45 g/liter, and preferably about20 to about 40 g/liter. In a preferred embodiment, the potassium cyanideis utilized in an amount generally from about 45 to about 95 g/liter,and preferably from about 50 to about 85 g/liter.

The thickness of the copper-tin-zinc alloy plating on cups 26, 226depends upon a number of factors including the current applied, currentdensity, plating time, weight of anode cups in vessel 110 to be plated,metals content of the plating bath, and flow rate of plating solutionthrough vessel 110. In one embodiment, plating time ranges from about 5minutes to about 70 minutes, desirably from about 15 minutes to about 50minutes, and preferably from about 20 minutes to about 35 minutes. Asdescribed herein, in a preferred embodiment of the present invention, itis desirable to utilize a variable current density process includingplating at two or more different current densities in order to provide aplated copper-tin-zinc alloy coating with desirable characteristics. Ina preferred embodiment, a copper-tin-zinc alloy layer or region isplated at a higher current density and a second copper-tin-zinc alloylayer or region is plated subsequently utilizing a lower current densityand preferably forms the exterior, exposed surface of the cup. Thevariable current density process can be performed in one or more platingbaths or solutions. In a preferred embodiment, the cups are plated withthe copper-tin-zinc alloy at a higher current density for a period oftime and then the current density is reduced to a second lower currentdensity, while the cups are maintained in the same solution, for anadditional period of time. Cathode current density for the relativelyhigher current density ranges generally from about 10 to about 1,076amps per square meter (1 to 100 amps per square foot). The highercurrent density is more preferably at least 16 amps per square meter(1.5 amps per square foot) and most preferably at least 26.9 amps persquare meter (2.5 amps per square foot). The higher current density ispreferably at most 107.6 amps per square meter (10 amps per square foot)and most preferably at most 86.1 amps per square meter (8 amps persquare foot). In one embodiment of the present invention, the relativelylower current density ranges generally from about 1.0 to about 27.0 ampsper square meter (0.1 to 2.5 amps per square foot). The lower currentdensity is preferably less than 21.5 amps per square meter (2 amps persquare foot). The lower current density is more preferably at least 4.3amps per square meter (0.4 amps per square foot) and most preferably atleast 6.4 amps per square meter (0.6 amps per square foot). The lowercurrent density is more preferably at most 16.2 amps per square meter(1.5 amps per square foot) and most preferably at most 12.9 amps persquare meter (1.2 amps per square foot). While the ranges for the highercurrent density plating and the lower current density plating providedoverlap, it is to be understood that the lower current density is alwayslower than the higher current density value utilized. As indicatedhereinabove, the resulting copper-tin-zinc alloy plating forms a layeror region having a surface portion exposed to the ambient atmospherewhen assembled in a finished cell that has a greater weight percentageof tin and a lesser weight percentage of copper than a lower, internalregion or layer of the copper-tin-zinc alloy plated coating.

Flow rate of the plating solution ranges generally from about 11.4 toabout 75.7 liters per minute, desirably from about 22.7 to about 56.8liters per minute, and preferably from about 30.3 to about 37.9 litersper minute measured at an in-line flow meter located between pump 108and vessel 110. The flow rates of other solutions, i.e. rinsingsolutions, alkaline cleaning solutions, acid etching or cleaningsolutions, passivating solutions, etc. utilized in the other processesdescribed, independently, can vary within the same ranges as the flowrate ranges described above for the plating solution. The platingcomposition is maintained during plating at a temperature generally fromabout 50° C. to about 70° C., desirably from about 56° C. to about 64°C., and preferably from about 60° C. to about 64° C.

The method for plating the cups 26, 226 preferably includes one or morepreplating steps performed before the copper-tin-zinc alloy platingstep, and one or more postplating steps performed after the cups 26, 226have been plated. The spouted bed electrode plating device is preferablyutilized in one embodiment to perform one or more of the preplating orpostplating steps in addition to performing the plating of the cups 26,226. In a first step, a predetermined amount of anode cups are loadedinto vessel 110. In one embodiment, vessel 100, including the cups 26,226, is moved from a first station of the spouted bed electrode platingdevice whereat a first process step is performed to at least a secondprocess station whereat a second process step is performed, wherein oneof the process steps involves plating the anode cup with acopper-tin-zinc alloy. In an alternative embodiment, the vessel 110 ismaintained at one station, such as described in U.S. Pat. No. 6,193,858herein incorporated by reference, and the copper-tin-zinc alloy platingprocess and at least one other process is performed at the station,preferably in sequence by cycling different treating solutions throughthe station.

The following cup processing steps are described with reference to thespouted bed electrode plating device having multiple stations, but it isto be understood that, as described, the same process steps can beperformed at a single station.

In a preferred embodiment, the negative electrode or counter electrodecups 26, 226 are processed in the vessel 110 of the spouted bedelectrode plating device by subjecting the cups to an alkaline cleaningprocess, after which the cups are rinsed, preferably with water, mostpreferably deionized water. The cups are then subjected to an acidcleaning or etching step followed by a further rinsing step. The cupscan then optionally be pre-treated with a cyanide solution (e.g., about8.22 g/liter (5 ounces/gallon) of water) pre-dip just prior to platingthe copper-tin-zinc alloy. Subsequently, the cups 26 are copper-tin-zincalloy plated and rinsed again. The cups are subsequently passivatedutilizing either an acid post-dip followed by a caustic post-dip or asalt post-dip, then rinsed, and subsequently dried.

The alkaline cleaning step is utilized to remove residue, if present, onthe surface of the cups. Accordingly, in one embodiment, the alkalinecleaning step is not performed, generally when the cups are relativelyclean. The cups 26, 226 are cleaned by circulating the cups 26, 226,such as described hereinabove with respect to copper-tin-zinc alloyplating, in vessel 110 utilizing an alkaline cleaning solution. In apreferred embodiment, ATOTECH™ 373 metal cleaner from Atotech USA Inc.of Rock Hill, S.C. is utilized as a cleaning agent. The ATOTECH™ 373 ismixed with water, preferably deionized water, in an amount generallyfrom about 14.64 to about 109.48 milliliters per liter (about 2 to about14 ounces per gallon), desirably from about 31.28 to 93.84 millilitersper liter (about 4 to about 12 ounces per gallon), preferably from about46.92 to about 78.20 milliliters per liter (6 to about 10 ounces pergallon), and most preferably 62.56 milliliters per liter (about 8 ouncesper gallon) of water to form a cleaning solution. The alkaline cleaningsolution is maintained at a temperature of generally from about 48.89°C. (120° F.) to about 60.0° C. (140° F.), and preferably from about48.88° C. (120° F.) to about 60.0° C. (140° F.). The alkaline cleaningstep is generally performed until the cups 26 have been cleaned to adesired degree, and generally from about 3 to about 5 minutes. Avibration device, such as described above, is utilized to vibrate thevessel 110 containing the cups 26, 226 in one embodiment. The vibrationdevice is preferably attached to vessel 110 and thus is transported fromstation to station. The vibration device is optionally, but preferably,used at each station, unless otherwise indicated.

After the alkaline cleaning step, the vessel 110 is transferred to arinsing station. Water, preferably deionized water, is utilized to rinsethe cups to remove any remaining alkaline cleaning solution to preventcarryover into the next step in the plating process. In the rinsingstep, the cups are circulated in vessel 110, such as describedhereinabove with respect to tin plating. In one embodiment, the cups 26,226 are rinsed in two separate tanks for a period of time ranginggenerally from about 10 to about 60 seconds per tank. Optionally, cups26, 226 in vessel 110 may be manually spray rinsed, preferably withagitation, in a separate tank before being processed in the rinse tank.The vibration device is preferably not utilized during the manual sprayrinsing.

The rinsed cups 26, 226 are transferred to an activation station whereinthe cups 26, 226 are circulated along with an activation solution withinvessel 110. The cup activation preferably utilizes an acid solution,containing an acid such as sulfuric acid. Other acids can be used.Preferably the acid is one that generates little gas and will not attackthe cup during activation. Preferably the acid anion is the same as thetin anion in the plating solution. The acid is present in an amountgenerally from about 6 to 14 weight percent, desirably about 8 to 12weight percent, and preferably about 10 weight percent based on thetotal weight of the solution. The activation step activates the surfaceof the cups 26, 226 for plating. The cups 26, 226 are circulated invessel 110 for a period of time ranging generally from about 1 to about10 minutes. The activation solution is maintained at a temperaturegenerally from about 12.78° C. (55° F.) to about 35° C. (95° F.), andpreferably from about 18.33° C. (65° F.) to about 35° C. (95° F.).

Subsequent to activation, the cups 26, 226 are preferably rinsed at afurther rinsing station in order to reduce carryover of activationsolution into the copper-tin-zinc alloy plating station. The anode cupsare preferably circulated in vessel 110 with water, preferably deionizedwater, preferably utilizing the vibration device, for a period of timeranging generally from about 10 to about 40 seconds.

After the rinse step, the copper-tin-zinc alloy plating process step isperformed as described hereinabove utilizing plating chamber 100.

After the cups 26, 226 have been plated with the copper-tin-zinc alloy,the anode cups are spray rinsed in vessel 110, in an appropriate tank,preferably with water utilizing a vibration device as describedhereinabove. Rinsing is generally performed for a period of time untilobservation of foaming ceases. Rinse times generally range from about 1to about 2 minutes.

In a further step, the cups 26, 226 are subjected to one or moresubsequent rinses in one or more tanks utilizing water, preferablydeionized water as described hereinabove, wherein the cups 26, 226 arecirculated in vessel 110. The purpose of the rinse is to remove anyplating solution and cyanide residue prior to a passivation step. Thevessel 110 is preferably vibrated during the one or more rinsing steps.The cups are rinsed for a period of time from about 60 to 90 seconds ineach tank.

In yet another step in the copper-tin-zinc plating process of cups 26,226 of the present invention, the plated cups 26, 226 are passivated ina passivation step. Several passivation processes can be used. In onepassivation process the cups are circulated in vessel 110 with apassivation solution containing an acid, such as 2 volume percentsulfuric acid. The cups 26, 226 are circulated in vessel 110 along withpassivation solution, preferably using the vibration device, for about10 to 30 seconds. The temperature of the passivation solution preferablyranges from about 17.78° C. (64° F.) to about 28.89° C. (84° F.), andmore preferably from about 23.33° C. (74° F.) to about 26.66° C. (80°F.). Following the acid passivation step, the cups 26, 226 arecirculated in vessel 110 with an additional passivation solutioncomprising a base such as, but not limited to, sodium hydroxide orpotassium hydroxide. The passivation solution is about 1.5 weightpercent RAYON® grade NaOH. The cups 26, 226 are circulated in vessel 110along with the passivation solution, preferably utilizing the vibrationdevice, for a period of time ranging generally from about 10 to about 30seconds. Temperature of the passivation solution ranges generally fromabout 17.78° C. (64° F.) to about 28.89° C. (84° F.), and preferablyfrom about 23.33° C. (74° F.) to about 26.66° C. (80° F.). Thepassivation step generally prevents discoloration of the copper-tin-zincalloy surface on the cup 26, 226. Afterwards, the passivation solutionis drained from the cups 26, 226. In an alternative passivation process,the cups are circulated in a vessel 110 with a passivation solutioncomprising a salt solution, such as a solution of about 50 g/liter ofMIRALLOY® Post Treatment Salt, for about 30 to 120 seconds, preferablyat about 17.78° C. (64° F.) to about 28.89° C. (84° F.), and morepreferably from about 23.33° C. (74° F.) to about 26.66° C. (80° F.).

After the cups 26, 226 are processed in the passivation step, the cups26, 226 are rinsed for a suitable period of time, preferably utilizingspray rinsing or water circulation, preferably deionized watercirculation, with the cups 26, 226 as described hereinabove. A vibrationdevice is preferably utilized during circulation of the anode cupswithin vessel 110. In a preferred embodiment, the cups 26, 226 are firstspray rinsed for about 1 to about 2 minutes, tank rinsed via circulationin vessel 110 for about 60 to 90 seconds, and further spray rinsed forabout 1 to about 2 minutes until substantially any remaining passivationsolution is removed and there is no observation of foaming in the rinse.

In a further step, the plated cups 26, 226 are dried utilizing forcedair and movement of the cups. Any suitable drying process can be used.For example, hot air can be forced through a spin dryer, a rotatingperforated drum, or an auger system. At a drying station according toone embodiment, heated air (e.g., from about 65.56° C. (150° F.) toabout 104.44° C. (220° F.) is forced through the inlet flange 112 ofvessel 110. In the drying process, internal deflector plate 120 isgenerally not utilized and the anode cups are dried by forced airmovement through vessel 110 and vibration of vessel 110 utilizing avibration device such as described hereinabove. The cups 26, 226generally remain at the drying station until dry with suitable dryingtimes ranging generally from about 4 to about 20 minutes, and preferablyfrom about 12 to about 20 minutes.

After the cups 26, 226 are dried, the same are removed from vessel 110and incorporated into the cells 10, 210 as described above.

EXAMPLE 1

Four lots of PR41 type zinc-air electrochemical battery cells were madeto evaluate the effects of hydrogen gassing. All lots were identicalexcept for plating of the anode cup, the type of zinc used and the typeof surfactant contained in the counter (negative) electrode. Thenegative electrodes contained 77.8 weight percent anode mix and 22.2weight percent electrolyte solution. The anode mix contained zinc, 0.25weight percent SANFRESH® DK-300 gelling agent, and 450 parts per millionby weight In(OH)₃. The electrolyte contained aqueous KOH (33 weightpercent KOH in water), 1 weight percent ZnO, and 100 parts per millionby weight DISPERBYK® 102 surfactant.

Post-plating of anode cups was done using the SBE process describedabove, with the alloy plating done at a single current density. Each lothad an anode cup formed from nickel-stainless steel-copper tricladstrip, with copper on the interior surface of the cup; a nickel platedsteel cathode can; an injection molded nylon gasket sealed between thecup and the can; a gelled electrolyte zinc negative electrode; and acatalytic (positive) electrode. The negative electrode contained zinc,potassium hydroxide electrolyte solution and gelling agent, as well aszinc oxide and indium hydroxide. The positive air electrode had anactive layer containing manganese oxide, carbon and a PTFE binder. Anickel expanded metal screen was embedded into the side of the activelayer facing the negative electrode, a layer of PTFE film pressurelaminated onto the surface of the active layer facing the bottom of thecan, two layers of polypropylene separator glued to the surface of theactive layer facing the negative electrode. A loose layer of PTFE filmwas located on the side of the air electrode facing the can bottom, witha loose layer paper between the loose layer of PTFE and the insidesurface of the can bottom. The anode cup plating, zinc type andsurfactant type for each lot is summarized in Table 1. ZCA 1230 zinc isa low-gassing zinc as described in U.S. Pat. No. 6,602,629 and isavailable from Zinc Corporation of America, Monaca, Pa., USA.

After the cells were assembled the holes in the can bottoms were sealedwith an adhesive tape, and the cells were aged for about 2 days beforetesting. Samples from each lot were tested on a sealed cell open circuitvoltage (SCOCV) test and a limiting current test, and the results aresummarized in Table 1.

The SCOCV test method was:

1. Seal the holes in the can bottoms with an adhesive tape.

2. Age the cells for 4 to 7 days, remove the tape.

3. Apply a layer of epoxy (e.g., HARDMAN® epoxy from Royal Adhesives andSealants of Bellville, N.J., USA, or DEVCON® epoxy from ITW Devcon ofDanvers, Mass., USA) to a tray, then press the cells into the layer ofepoxy with the can bottom facing the tray to seal the holes in the canbottoms and prevent any additional air from entering the cells throughthe holes.

4. Store the epoxy-sealed cells for 7 days at 45° C. and then test theopen circuit voltage (OCV) at room temperature. The average OCV's areshown in Table 1.

The limiting current was measured by storing cells sealed with adhesivetape for 4 weeks at 71° C., removing the tape, and discharging the cellsat room temperature at a constant voltage of 1.1 volts using a variableresistor and measuring the current at 60 seconds. The limiting currentwas also measured after storing cells sealed with adhesive tape for 28days in an oxygen atmosphere (about 0.136 to 0.340 atmospheres (2-5pounds per square inch)) with about 40 to 70 percent relative humidity.The average limiting currents after storage for 7 days at 45° C. and for28 days in an oxygen atmosphere are shown in Table 1.

The results show that plating the anode cups with a copper-tin-zincalloy is superior to both no plating and plating with tin, and thecombination of plating with a copper-tin-zinc alloy and NGBIA type zincprovides superior sealed cell open circuit voltage and limiting current.

TABLE 1 Anode Cup SCOCV (volts) Limiting current (mA) Plating Zinc TypeSurfactant 7 days/45° C. 4 weeks@71° C. 28 days in O₂ none ZCA 1230CARBOWAX ® 0.99 4.80 4.20 550 Sn ZCA 1230 CARBOWAX ® 1.03 4.00 0.80 550Cu—Sn—Zn ZCA 1230 CARBOWAX ® 1.02 5.00 1.10 550 Cu—Sn—Zn NGBIADISPERBYK ® 1.05 7.20 4.70 115 D102

EXAMPLE 2

Three lots of hydrogen generating button cells with a diameter of about11.6 mm and a height of about 5.4 mm were made. The lots were identicalexcept for the cup and the types of zinc and surfactant used in thecounter electrode, as shown in Table 2. The process described above wasused to plate the cells; for one plated anode cup lot, the cups werepassivated using a post-treatment used, and for the other an acid-basepassivation was used. The cells in both lots were otherwise like thosein Example 1, except the loose layer of PTFE film was replaced with ahydrogen-permeable sintered PTFE material having very low oxygenpermeability. Gas generation was substantially lower with both lots withCu—Sn—Zn plated cups, NGBIA zinc and D102 surfactant.

After the cells were assembled they were aged on adhesive tape for abouttwo weeks before testing. Sample cells from each lot were tested on agassing test, and the average amount of gas generated, in microliters(μl), is shown in Table 2.

The gassing test method used was:

1. Remove the adhesive tape from the bottom of a cell.

2. Place the cell into a small (e.g., 6.0 cm by 5.5 cm) aluminum foillaminated poly bag, remove most of the air from the bag, and heat sealthe bag closed.

3. Place the sealed bag into a glass container and fill the containerwith oil (e.g., a vacuum oil), completely submersing the bag in oil.

4. Close the glass container with a graduated glass tube (e.g., agraduated pipette) so that no air bubble remains in the container andthe oil rises part way up the graduated glass tube; record the level ofthe oil in the tube.

5. Immerse the closed glass container with graduated glass tube in awater bath at 60° C.

6. Check the level of the oil in the graduated glass tube at 14 days,maintaining the later level in the water bath at a constant level duringthe test, and record the volume of oil displaced since beginning thetest; this is the volume of gas generated during the test.

TABLE 2 Gas Anode Generated Cup Plating Zinc Type Surfactant Passivation(μl) none ZCA 1230 CARBOWAX ® 0.0726 550 Cu—Sn—Zn NGBIA 115 DISPERBYK ®salt 0.0128 D102 Cu—Sn—Zn NGBIA 115 DISPERBYK ® acid-base 0.0014 D102

In the embodiments above, the cell is a button type cell; however, othercell sizes and shapes can be used. For example, the cell can be a smallprismatic cell, with a generally rectangular or square cross sectionalshape having maximum length and width dimensions that allow properpost-plating of the cups using the preferred SBE process. In general,for button and prismatic cells, the maximum cell height is preferably nogreater than about 10 mm, more preferably no greater than about 8 mm,and the maximum width dimension (perpendicular to the height) is no morethan about 30 mm, more preferably no more than about 20 mm, and evenmore preferably no more than about 15 mm, and most preferably no morethan about 11.6 mm. For larger size cells, with larger cups, otherplating processes, such as a rack plating process, can be used forpost-plating the cups.

The embodiments described above are hydrogen generating cells. Othertypes of electrochemical cells with aqueous alkaline electrolytes can beused, such as oxygen generating cells, air depolarized battery cells andeven other types of battery cells (e.g., zinc/manganese dioxide andzinc/silver oxide cells), in which one part of the cell housing is incontact with and serves as the current collector for the counterelectrode. Other components and materials would be recognized assuitable by those skilled in the art. For example, other active,catalytic, electrically conductive and binding materials can be used inthe electrodes, other separator materials can be used, and otheradditives to the electrodes and electrolytes can be used, based on therequirements and limitations of the electrochemical cell type. In oxygengenerating cells and air depolarized battery cells the diffusion layerbetween the catalytic electrode and the can bottom will be morepermeable to air, such as a polytetrafluoroethylene film membrane.

EXAMPLE 3

Copper-tin-zinc alloy plated anode cups for PR41 size (7.9 mm diameter,3.6 mm high, zinc-air) button cells were made using the samepost-plating process used in Example 1 but with the alloy plating stepdone at either a single current density or a combination of twodifferent current densities. One batch of approximately 4465 cups (LotA) was made with an alloy plating current density of 43 amps per squaremeter (4.0 amps per square foot) for a time corresponding to 14amp-hours, and another batch of approximately 4465 cups (Lot B) was madewith a two stage alloy plating step in which the current density was 43amps per square meter (4.0 amps per square foot) for a timecorresponding to 14 amp-hours, followed by a current density of 8.6 ampsper square meter (0.8 amps per square foot) for a time corresponding to1 amp-hour.

An aqueous KOH solution (33 weight percent KOH) was applied to theexternal surface of the central portion of samples of cups from Lots Aand B. The test was repeated several times. Each time, within 30 minutesabout 30 to 100 percent of the cups from Lot A were tarnished in thearea to which the KOH solution had been applied, while none of the cupsfrom Lot B were discolored.

Sample cups from Lots A and B were stored at 60° C. and 90 percentrelative humidity. After 4 days all of the samples from Lot A werediscolored, while none from Lot B were discolored.

Sample cups from Lots A and B were stored at room temperature andambient humidity for about 6 weeks. Some of the samples from Lot A werediscolored in the central portion of the outside of the cups and otherswere not. None of the samples from Lot B were discolored. The cups aremore susceptible to discoloration in the central flat area on theoutside of the cups, where the current density was highest duringplating and the copper content tends to be highest.

These tests show that cups with a higher copper content in the surfaceportion of the plated alloy layer are more susceptible to discolorationand that plating the alloy using a variable current density (lower atthe end of the alloy plating step) can provide a lower copper content inthe surface portion of the alloy layer that is whiter and more resistantto discoloration, while the bulk of the plated alloy layer is platedmore quickly, at a higher current density, and still provide an alloycomposition that has good resistance to cell internal gassing.

EXAMPLE 4

Copper-tin-zinc alloy plated cups for AC13 size button cells weremanufactured utilizing the same post-plating process utilized in Example1, with the proviso that the alloy plating step was performed at eithera single current density or a combination of different current densitiesas indicated in Table 4 hereinbelow. The plating was performed on anickel/stainless steel/copper cup substrate with copper being present onthe inside surface of the cup prior to plating. Each lot contained 100cups. The cups of each lot after plating were assembled into cells andthe cells stored at 60 C and 90 percent relative humidity for 50 days.The exposed exterior surfaces of the cups of the cells were visuallyinspected after the 50 days and the results of the inspection are setforth in Table 4. Undiscolored cups generally maintained a substantiallysilver-colored appearance, and discolored cups had a substantially blackor dark appearance.

TABLE 4 Current Density Characteristics of % of Cups Having LotCopper-Zinc-Tin-Alloy Plating Discoloration 1  8.6 A/m² (0.8 ASF) for~15 AH 90 2 16.1 A/m² (1.5 ASF) for ~15 AH 70 3 32.3 A/m² (3.0 ASF) for~15 AH 50 4   43 A/m² (4.0 ASF) for ~15 AH 40 5 43 A/M² (4.0 ASF) for 14AH followed  0 by 8.6 A/m² (0.8 ASF) for ~1 AH

The results set forth in Table 4 show that anode cups plated with acopper-tin-zinc alloy using a dual current density process weresignificantly more resistant to discoloration than cups plated at asingle current density. It is further shown that the dual currentdensity plating profile also outperformed the lots plated at singlecurrent densities that were utilized in the dual current density platingprocess.

It will be understood by those who practice the invention and thoseskilled in the art that various modifications and improvements may bemade to the invention without departing from the spirit of the disclosedconcept. The scope of protection afforded is to be determined by theclaims and by the breadth of interpretation allowed by law.

What is claimed is:
 1. A method for forming a coated casing for anelectrochemical cell, comprising the steps of: providing an electrodecasing comprising a metal substrate having an interior surface and anexterior surface, the substrate comprising a copper layer on theinterior surface; plating a first metal layer with a plating solution onthe interior surface and on the exterior surface of the substrateutilizing a first current density; and changing the first currentdensity to a second different current density while the substrate of theelectrode casing is in contact with the plating solution and plating asecond metal layer on the first metal layer on the interior surface andon the first metal layer on the exterior surface, the second metal layerbeing adapted to be exposed to the atmosphere when the electrode casingis assembled in an electrochemical cell; wherein: the first metal layerhas a ratio of copper from 50 to 70 weight percent, tin from 26 to 42weight percent, and zinc from 3 to 9 weight percent, measured at acentral portion of the exterior surface of the substrate of theelectrode casing by SEM/EDS; and the second metal layer has a ratio ofcopper from 36 to 46 weight percent, tin from 42 to 57 weight percentand zinc from 6.5 to 9.5 weight percent, measured at a central portionof the exterior surface of the substrate of the electrode casing bySEM/EDS.
 2. The method according to claim 1, wherein the first currentdensity of the plating ranges from 16 to 107.6 amps per square meter,and wherein the second current density of the plating ranges from 4.3 to21.5 amps per square meter with the proviso that the second currentdensity is lower than the first current density.
 3. The method accordingto claim 1, wherein after plating the first metal layer and the secondmetal layer, a passivating step is performed on the plated electrodecasing utilizing salt passivation or an acid-base passivation.
 4. Themethod according to claim 1, wherein the plating is performedelectrolytically utilizing a rack plating device, a barrel platingdevice, a spouted bed electroplating device or a vibratory platingdevice.
 5. The method according to claim 4, wherein the plating isformed electrolytically utilizing a spouted bed electroplating device.6. The method according to claim 1, wherein the first current density ofthe plating is at least 26.9 amps per square meter and at most 107.6amps per square meter, and wherein the second current density of theplating is at least 4.3 amps per square meter and at most 16.2 amps persquare meter.
 7. The method according to claim 1, wherein the secondlayer has a ratio of surface values measured according to XPS of 73 to84 atomic percent copper, 11 to 17 atomic percent tin, and 4 to 10atomic percent zinc.
 8. A method for forming a coated casing for anelectrochemical cell, comprising the steps of: providing an electrodecasing comprising a metal substrate having an interior surface and anexterior surface wherein from the exterior surface to the interiorsurface the substrate has a nickel layer, a stainless steel layer and acopper layer; plating a first metal layer with a plating solution on atleast the exterior surface of the Substrate utilizing a first currentdensity; and changing the first current density to a second differentcurrent density while the electrode casing is in contact with theplating solution and plating a second metal layer that is adapted to beexposed to the atmosphere when the electrode casing is assembled in anelectrochemical cell on the first layer; wherein: the first and secondplated layers each comprise copper, tin and zinc; the first layer has aratio of surface values according to XPS of 85 to 91 atomic percentcopper, 6 to 11 atomic percent tin, and 2 to 6 atomic percent zinc; andthe second layer has a ratio of surface values measured according to XPSof 73 to 84 atomic percent copper, 11 to 17 atomic percent tin, and 4 to10 atomic percent zinc.
 9. The method according to claim 8, wherein thecoated casing is used in an electrochemical cell having a firstelectrode and a second electrode, wherein the first electrode is anegative electrode comprising zinc and the second electrode is apositive electrode comprising at least one of manganese dioxide, asilver oxide and a catalytic material for reducing oxygen or forgenerating hydrogen or oxygen, wherein the cell is a button cell, andwherein the first layer has a thickness greater than a thickness of thesecond layer, and wherein the first electrode is free of added mercury.10. A method for forming an electrochemical cell, comprising the stepsof: providing a first electrode casing comprising a metal substratehaving an interior surface and an exterior surface; plating the interiorsurface and the exterior surface of the substrate of the first electrodecasing with a copper-tin-zinc alloy utilizing a variable current densitywhile the substrate is in contact with a plating solution therebyproducing a plated substrate having a first layer and a second layer,and wherein a tin content increases from the first layer to the secondlayer; and forming an electrochemical cell comprising a first electrodein contact with the interior surface of the plated substrate of thefirst electrode casing and an aqueous alkaline electrolyte, wherein,prior to plating, the substrate of the first electrode casing has acopper layer on the interior surface; wherein: the current density isdecreased during the plating such that the first layer has a ratio ofcopper from 50 to 70 weight percent, tin from 26 to 42 weight percent,and zinc from 3 to 9 weight percent measured at a central portion of theexterior surface of the plated substrate by SEM/EDS, and the secondlayer has a ratio of copper from 36 to 46 weight percent, tin from 42 to57 weight percent and zinc from 6.5 to 9.5 weight percent measured at acentral portion of the exterior surface of the plated substrate bySEM/EDS.
 11. The method according to claim 10, wherein a first currentdensity of the variable current density plating ranges from 16 to 107.6amps per square meter, and wherein a second current density of thevariable current density plating ranges from 4.3 to 21.5 amps per squaremeter with the proviso that the second current density is lower than thefirst current density.
 12. The method according to claim 11, wherein thefirst current density of the plating is at least 26.9 amps per squaremeter and at most 107.6 amps per square meter, and wherein the secondcurrent density of the plating is at least 4.3 amps per square meter andat most 16.2 amps per square meter.
 13. The method according to claim10, wherein the plating is performed electrolytically utilizing a rackplating device, a barrel plating device, a spouted bed electroplatingdevice or a vibratory plating device.
 14. The method according to claim13, wherein the plating is formed electrolytically utilizing a spoutedbed electroplating device.
 15. The method according to claim 10, whereinprior to plating from the exterior surface to the interior surface thesubstrate has a nickel layer, a stainless steel layer, and said copperlayer.
 16. The method according to claim 10, wherein the second layerhas a ratio of surface values measured according to XPS of 73 to 84atomic percent copper, 11 to 17 atomic percent tin, and 4 to 10 atomicpercent zinc.
 17. The method according to claim 10, wherein the firstelectrode is a negative electrode comprising zinc and theelectrochemical cell further comprises a second electrode, wherein thesecond electrode is a positive electrode comprising at least one ofmanganese dioxide, a silver oxide and a catalytic material for reducingoxygen or for generating hydrogen or oxygen, wherein the cell is abutton cell, and wherein the first layer has a thickness greater than athickness of the second layer, and wherein the first electrode is freeof added mercury.